This invention relates generally to systems for driving frequency converters, and more particularly, to a system wherein the frequency converter is provided with a predetermined signal which corresponds to a desired current at a load which is connected to the outputs of the frequency converter.
It is customary in the theoretical analysis of frequency converters and their connected loads to correlate the respective phase currents and voltages with two-dimensional current or voltage load vectors. In situations where a frequency converter is energized by means of predetermined DC input voltage wherein the switches of the converter selectably connect the respective phase outputs of the converter to the respective polarities of the DC input voltage supply such that the DC input supply is connected to the output of the converter selectably at its positive or negative terminals, each such phase output of the converter can assume only one of two discrete voltages which are defined by the magnitude of the DC input voltage and the state of the drive signal. In the operation of such an arrangement, a finite number of combinations are possible for driving the converter switches. In a three-phase converter, only 2.sup.3 =8 switch combination states are possible, thereby allowing only a finite number of possible states of the resulting two-dimensional voltage vector. The two switch combination states where the phase outputs of the converter are all connected to either the positive or negative DC voltage input such that all of the phase outputs are in the same state provide the zero voltage point of the resulting two-dimensional voltage vector. The remaining switch combinations correspond to discrete two-dimensional voltage vectors.
In a three-phase arrangement, a total of 6 discrete vectors, Z1 to Z6, and 2 zero state (same state condition) vectors, Z.sub.+ and Z.sub.-, are therefore possible for the two-dimensional voltage vector. Intermediately positioned vectors can be provided in this arrangement by alternatingly addressing adjacent ones of the two-dimensional voltage vectors, thereby producing a pulsed output voltage. The mean value of the pulsed output voltage corresponds to the voltage value of the corresponding phase output. Thus, during pulsed operation of the converter, the load voltage is controlled, while the load current is free to assume a value which corresponds to the nature of the load.
It is often desirable in the operation of a load, illustratively a rotating-field machine, to regulate the load current to predetermined values. One known arrangement for achieving a regulated load current utilizes hysteresis controllers for determining a control difference between the actual current flowing through a load and a desired load current value. In such an arrangement, the control difference is determined for each phase current and the two-dimensional voltage vector of the load ("voltage vector") is controlled as a function of the control differences. In this manner, switching is performed from one discrete vector to another. In the known hysteresis controlled arrangement, the state of the output signal always changes if the control difference is outside of a range which is established by positive and negative tolerance limits. With each such state change, the polarity of the voltage is reversed at the respective phase output such that a further one of the discrete vectors which is possible for the voltage vector is addressed. Since the hysteresis controllers operate independently of one another for each phase output, the particular voltage vector which is addressed in each case is generally not predeterminable. Also, randomly occurring same state conditions are possible wherein all of the phase outputs are in the same state. It is a further problem with this known arrangement that a phase current can deviate from the preset value by as much as twice the value of the tolerance width, notwithstanding that switch-overs occur if a tolerance limit is exceeded.
It is, therefore, an object of this invention to provide an improved method for forming control signals for a pulsed converter wherein load current is controlled.